Light-emitting diodes (hereinafter also referred to as LEDs) having a light-emitting layer which is composed of an aluminum gallium indium phosphide mixed crystal (compositional formula: (AlXGa1-X)YIn1-YP, 0≧X≧1) and which is vapor-grown on an n-type or p-type gallium arsenide (GaAs) single-crystal substrate are known to emit light having a wavelength corresponding to green light to red light (see, for example, Non-Patent Document 1).
Specifically, an LED having on a GaAs substrate a light-emitting layer composed of an aluminum gallium indium phosphide mixed crystal ((AlXGa1-X)0.5In0.5P: 0≦X≦1) (Y=0.5 in the above compositional formula) is employed.
In an LED having a light-emitting layer composed of (AlXGa1-X)YIn1-YP, in order to attain high emission intensity, diffusion of device operation current over a wide area of the light-emitting layer and effective extraction of light to the outside are essential. Thus, a current diffusion layer and a window layer are generally provided on the light-emitting layer.
The window layer allows light emitted from the light-emitting layer to transmit to the outside. For example, an LED having a window layer composed of gallium phosphide (GaP) is disclosed (see Patent Document 1).
In an LED having a stacked structure vapor-grown on a GaAs substrate, light emitted from the light-emitting layer can be extracted only from the upper side of the LED, since the GaAs substrate is not transparent with respect to emission wavelength. Thus, efficiency of extracting light to the outside is not satisfactory, which is to be improved.
In order to solve the problem, a method for producing an LED has been proposed. In the method, a substrate which is transparent with respect to emission wavelength is joined onto a stacked structure formed on a GaAs substrate, and the GaAs substrate provided for vapor-growth of the stacked structure is removed.
By virtue of the thus-joined substrate which is transparent with respect to emission wavelength, the LED produced through the above method allows light emission from the upper side as well as from the backside and side planes, thereby attaining high light extraction efficiency.
There have been known such methods for producing an LED including joining, onto a stacked structure having a light-emitting layer, a semiconductor substrate which is transparent with respect to emission wavelength (e.g., GaP, zinc selenide (ZnSe), or silicon carbide (SiC)) (see, for example, Patent Documents 2 and 3).
Another disclosed technique for producing an LED includes joining, onto a stacked structure, a GaP substrate which is transparent with respect to emission wavelength by the mediation of a transparent conductive film such as indium tin complex oxide film (ITO) (see, for example, Patent Document 4).
[Non-Patent Document]
Y. Hosokawa, Journal of Crystal Growth (Holland), 2000, Vol. 221, p. 652-656
[Patent Document 1]
U.S. Pat. No. 5,008,718 specification
[Patent Document 2]
Japanese Patent No. 3230638
[Patent Document 3]
Japanese Patent Application Laid-Open (kokai) No. 2001-244499
[Patent Document 4]
Japanese Patent No. 2588849